1. Field of the Invention
The present invention relates to an image processing apparatus that detects edges of characters in an image with accuracy to increase the image quality of the characters.
2. Discussion of the Background
In a known image processing method of extracting character areas of an image on the basis of edge distribution information of the image, a plurality of band-limited image data is generated from image data of an image. The edges of characters in the image are detected based on at least one of the plurality of band-limited image data, and character areas of the image are extracted based on edge distribution information of the image.
Also, an apparatus is known in which it is determined whether or not selected picture elements include edge areas based upon the continuity of differences in the densities of surrounding picture elements, and the method of binarization is switched based upon the result of the determination.
However, in the above-described method, an image formed of halftone dots and having a relatively small number of lines is detected to be an edge component, so that an operation to extract character areas may fail. In the above-described apparatus, if characters are blurred or broken such that the outline portions of the characters are uneven, or if any outline portions of the characters fall within an area having halftone dots so that the outline portions of the characters are discontinuous, determination of these characters is hard.
In wavelet conversion, generally, sub-sampling (omitting certain picture elements) is performed. In a processing system which performs sub-sampling, the higher the layer of hierarchy of the image data, the smaller the size of the image data (i.e., the resolution is lower). Therefore, by using image data of a relatively higher layer of hierarchy, area determination can be realized with relatively small-scale hardware.
However, if most of the characteristic features of a character are contained in the high frequency components of the character which have been omitted by sub-sampling, the characteristic features of the character are destroyed, so that the accuracy of the area determination is decreased.